Global patterns of colouration complexity in the Paridae: Effects of climate and species characteristics across body regions

1. Avian plumage colouration is an iconic example of trait variability among species. Sexual, social and natural selection, and the environmental variables modulating them are the main drivers of this variability. 2. So far, most research exploring environmental effects on the variability of plumage colouration has focused on the variation in overall plumage darkness. Research on other aspects of colour variation, such as the diversity of colours exhibited by a species (i.e. colour complexity), is limited and has produced inconsistent results. Furthermore, colour complexity has mostly been analysed at the whole-plumage level, despite the possibility that the colour complexity of different plumage patches may be sensitive to different environmental factors. 3. Here, we quantify male and female colouration in 58 species of the family Paridae, and use multi-predictor Bayesian phylogenetic mixed models to estimate the relationship of colouration with biotic and climatic variables that quantify environment, and with several species-specific characteristics. We consider both the colouration of the whole plumage and the colouration of four separate colour patches (head, chest, back and wing). 4. We find that Paridae species in climates with intermediate temperatures present more complex colouration than do species in warmer/colder climates. In addition, males, relatively small species, and species with relatively greater sexual dichromatism have more complex plumage colouration. We find that the numbers of predators and sympatric conspecifics are more associated with female colouration than with male colouration. Finally, the strength of the associations with colour complexity is specific to each plumage region: species recognition, beak size and climate variables related to competition for reproductive resources (i.e. precipitation seasonality) are more strongly associated with colouration complexity of the head and breast than with that of the back and wing. 5. Overall, our results illustrate the importance of climatic and social variables, the link between colour complexity and dichromatism in both sexes, and the analysis of distinct plumage areas for understanding global patterns of colouration complexity and the processes that promote them.

We thank H. van Grouw, M. Adams and A. Bond from the Bird Group at the Natural History Museum at Tring for providing access to and expertise in the collection used in this study. We also thank A. Estrada, D. G\u00F3mez, E. Harscouet, N. Merino\u2010Recalde and A. Fargevieille for their technical, analytical and conceptual help and U. Johansson for sharing the Paridae tree used in our analyses. We appreciate the work and time of the three anonymous reviewers who helped to improve previous versions of this manuscript. This work was funded by a Synthesis+ grant (GB\u2010TAF\u2010TA3\u2010027 20) from the European Commission granted to D.L.\u2010I. D.L.\u2010I. was funded by a postdoctoral grant from the Basque Government Department of Education (POS\u20102019\u20101\u20100026).